The present invention relates to fluid displacement apparatus and more particularly to a scroll-type machine especially adapted for compressing gaseous fluids and having means for varying the displacement and hence capacity thereof.
A class of machines exists in the art generally known as "scroll" apparatus for the displacement of various types of fluids. Such apparatus may be configured as an expander, a displacement engine, a pump, a compressor, etc. The present invention, however, is particularly applicable to compressors, and therefore for purposes of illustration is disclosed in the form of a gaseous fluid compressor.
Generally speaking, a scroll apparatus comprises two spiral scroll wraps of similar configuration each mounted on a separate end plate to define a scroll member. The two scroll members are interfitted together with one of the scroll wraps being rotationally displaced 180 degrees from the other. The apparatus operates by orbiting one scroll member (the "orbiting" scroll member) with respect to the other scroll member (the "fixed" scroll member) to make moving line contacts between the flanks of the respective wraps defining moving isolated crescent-shaped pockets or chambers of fluid. The spirals are commonly formed as involutes of a circle, and ideally there is no relative rotation between the scroll members during operation, i.e., the motion is purely curvilinear translation (i.e. no rotation of any line in the body). The fluid pockets carry the fluid to be handled from a first zone in the scroll apparatus where a fluid inlet is provided, to a second zone in the apparatus where a fluid outlet is provided. The volume of a sealed pocket progressively changes as it moves from the first zone to the second zone. At any one instant in time there will be at least one pair of sealed pockets, and when there are several pairs of sealed pockets at one time, each pair will have different volumes. In a compressor the second zone is at a higher pressure than the first zone and is physically located centrally in the apparatus, the first zone being located at the outer periphery of the apparatus.
Two types of contacts define the fluid pockets formed between the scroll members: axially extending tangential line contacts between the spiral faces of the wraps caused by radial forces ("flank sealing"), and area contacts caused by axial forces between the plane edge surfaces (the "tips") of each wrap and the opposite end plate ("tip Sealing"). For high efficiency, good sealing must be achieved for both types of contacts. In a conventional scroll compressor (i.e. one in which the wraps are involutes of a circle) good flank sealing requires that there be no relative rotation between the scrolls.
The concept of a scroll-type apparatus has been known for some time and has been recognized as having distinct advantages. For example, scroll machines have high isentropic and volumetric efficiency, and hence are relatively small and lightweight for a given capacity. They are quieter and more vibration free than many compressors because they do not use large reciprocating parts (e.g. pistons, connecting rods, etc.), and because all fluid flow is in one direction with simultaneous compression in plural opposed pockets there are less pressure-created vibrations. Such machines also tend to have high reliability and durability because of the relative few moving parts utilized, the relative low velocity of movement between the scrolls, and an inherent forgiveness to fluid contamination.
The capacity, or displacement per revolution, of a scroll machine is in part a function of the orbiting radius of the orbiting scroll, which in turn is in part a function (inverse) of wrap thickness; i.e., the greater the wrap thickness the smaller the orbiting radius and capacity of the machine, all other things being equal. The present invention resides in the discovery of a relatively simple technique for varying wrap thickness an thereby varying the displacement or capacity of the machine, even while the machine is operating. The machine of the present invention is therefore ideally suited for applications where capacity modulation is desirable. Furthermore the machine of the present invention is capable of providing infinitely variable modulation, from almost zero displacement to full displacement, while maintaining full displacement efficiency throughout the entire displacement range. The present machine offers the additional advantage that friction and wear are reduced as capacity is reduced, and yet full lubrication is provided at all displacements.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.